STIRred AND SHAKEN
owl writes:
https://computer.rip/2023-08-07-STIRred-AND-SHAKEN.html
In a couple of days, I pack up my bags to head for DEFCON. In a rare moment of pre-planning, perhaps spurred by boredom, I looked through the schedule to see what's in store in the world of telephony. There is a workshop on SS7, of course [1], plenty of content on cellular, but as far as I see nothing on the biggest topic in telecom security: STIR/SHAKEN.
I can venture a guess as to why: STIR/SHAKEN is boring. So here we go!
The Nature of Circuit Switching
Understanding today's robocalling problem requires starting a long time ago. Taking you all the way back to the invention of the telephone would be a little gratuitous, but it is useful to start our discussion with the introduction of direct distance dialing in 1951. In that year, the first long-distance call was completed based only on the customer dialing a number. Over the following decades direct distance dialing became more common and fewer telephone users had to speak to an operator to have a long-distance call established. Today, it's universal.
Handling dial calls over long distance trunks is a bit complicated, though. For local calls, handling was relatively simple. The other customer was connected to the same exchange that you were, so the exchange just needed to be able to detect your dialing and select the correct local loop corresponding to the number you dialed. Step-by-step (SxS) switches have been handling this problem since the turn of the 20th century. For long distance calls, though, the recipient will not be on the same switch---they'll be on a foreign exchange.
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